Recent years have seen the increasing availability of frequency converters with a Slim DC-Link, apparently offering a lower harmonics signature. This solution is based on a low internal DC-bus capacitance. Harmonic currents are mostly generated while the filter capacitors are being charged and the level of harmonic currents depends on filter capacitance, therefore filters with low capacitance generate lower levels of harmonics. Taking this low-cost approach, manufacturers dramatically reduce the capacitance of the DC link and even without a choke, this reduces the harmonic current to a Total Harmonic Distortion (THDi) of less than 40%. However, it shifts mains interference to a higher frequency spectrum which is not covered by the definition of THDi. A new upcoming standard is aimed to remedying this fact. It defines a partial weighted harmonic distortion (PWHD) where it can be seen from the above graph that Slim DC link have even higher PWHD.
PWHD is part of IEC61000-3-12 regulations and Slim DC Link does often not comply. Due to the broad frequency spectrum of Slim DC-Link devices, there is therefore an increased risk of resonance with other components connected to the mains, such as fluorescent lamps or transformers. The argument for the use of small capacitors in the DC-link is mainly because inverters of this construction have a lower harmonics signature.
Reducing the DC-link capacitance facilitates the adoption of film capacitors instead of electrolytic capacitors without compromising the reliability of the capacitor bank. Film capacitors are useful not only because of their longevity but also because their lower heat loss eases the thermal management of the drive. Nevertheless, the main advantage is the reduced harmonic content of the line currents without the need for inductive elements, reducing total harmonic distortion. More specifically, it reduces the THDi (THC: Total Harmonic Current/IEC61000-3-12 ed.2 2011) level down to 30-35% in the range up to 2kHz. However, the current distortion is just moved to a higher frequency range and therefore the mains interference higher than 2kHz would be much higher and especially at the switching frequency it will be dominating. In any case, conventional drives with reduction of the THC below 45% comply with the International and European harmonics standard IEC/EN61000-3-12 which specifies the limits for harmonic currents created by equipment connected to public low-voltage supply systems. Furthermore, part of IEC61000-3-12 standards, has now started a new work for emission requirements due to serious disturbances of smart meters used on the mains supply in the frequency range 2-150kHz.
Additionally, VSDs with Slim DC-Links exhibit serious weaknesses on the load side. With converters of this sort, load variations result in significantly increased voltage variations. Consequently, they have a greater tendency to oscillate in response to load variations on the motor shaft. A small DC-link capacitor does not provide satisfactory reduction in the harmonics caused by the input diode rectifier, which results in unwelcome fluctuation of the DC link voltage. This introduces noise and vibration as well as torque ripple on the motor shaft and thus the drive becomes very sensitive to rapid speed changes, in turn causing large DC-link over-voltages that can result in the frequency converter tripping. When load shedding, such as during rapid deceleration, the motor acts as a generator, raising the voltage level of the dc link. In response to this, devices with slim DC links shut down faster than conventional devices in order to protect against destruction due to overload or overvoltage. Because of their small or zero capacitance, Slim DC-Link converters are also poorly equipped to ride through mains dropouts. Generally, a Slim DC-Link has around 10% of the capacitance of a conventional DC link. In addition to mains interference due to the input current, converters with Slim DC-Link also pollute the mains with the switching frequency of the motor-side inverter. This is clearly visible on the mains side due to the low or zero capacitance of the DC link.
Strengths and weaknesses
The strengths and weaknesses of using Slim DC-Link are:
- Lower cost
- Part of drive
- Low harmonics in frequency range below 2 KHz
- Impact on drive motor: torque ripples, high temperature, and noise and vibration
- Low system efficiency
- Short motor cable
- Risk of system/grid resonances/trips
- High harmonics above 2 KHz
- Adherence of PWHD part to IEC61000-3-12 and future regulations of frequency standardization TC77A
Film capacitors versus electrolytic capacitors
The reduced DC-link capacitance of ‘Slim’ VSDs means that there is a relatively high ripple voltage in the DC-link compared to a conventional VSD with DC-link chokes. Ripple voltage across a capacitor increases the internal heat generation and therefore can reduce the lifetime of the capacitor. Film capacitors had not previously been suitable for use as VSD DC-link capacitors because they were not available at suitably high capacitance values and voltage ratings. However, recent developments in film capacitor technology mean they are now available at suitable voltage ratings for use as dc link capacitors.
Slim versus conventional DC link VSDs
A Slim DC-Link VSD with a 6-pulse uncontrolled rectifier can slow the rise time and extend the duration of input current peaks because of the low DC link capacitance. The capacity to store charge is also much reduced so that for the same motor load, the input rectifier has to conduct for longer to provide the necessary current to the motor. A Slim DC-Link design therefore results in lower levels of harmonic current distortion and RMS input current with levels comparable with those of a conventional VSD with DC-link chokes. The manufacturers of Slim DC-Link VSDs typically claim the THDi is in the region of 30% – 35% although this is very dependent on the mains supply impedance, in the same way that similar levels can be achieved by conventional VSDs with DC-link chokes dependent on the mains supply impedance.
Furthermore, the relatively small DC-link capacitance of a Slim DC-Link VSD potentially makes it sensitive and therefore more likely to trip, in a number of typical application operating conditions. Due to the relatively small DC-link capacitance the ripple voltage in the DC-link is much higher than that of a conventional VSD with DC chokes driving a motor operating at full load. Under normal full load operating conditions a conventional VSD with DC chokes would typically have a ripple voltage of less than 5% peak to peak, which is approximately one-third of the typical ripple voltage of a Slim DC-Link VSD. However, depending of the design of the Slim DC-Link VSD, it may impact on the performance of the motor and will generally make the VSD more sensitive when operating under abnormal or dynamic conditions.
The harmonic currents and consequent harmonic voltage distortion in the mains supply of a 6-pulse PWM VSD can be a problem if the harmonic currents are excessive, as stated earlier. However, harmonic currents can be limited by relatively simple measures and therefore this should not deter the use of VSDs in, e.g. HVAC systems, as they offer many benefits, not least improved control of the conditioned space at lower energy consumption compared to conventional methods. However, when VSDs are used extensively throughout a building’s HVAC system, the use of multiple VSDs without a harmonic filter may result in operational problems and the need to oversize cables, switchgear and transformers. The use of VSDs with simple harmonic filtering such as DC-link chokes or AC input chokes will generally prevent harmonic problems. In order to reduce harmonic current emissions from a VSD, the current pulses should be limited. This is achieved through the use of chokes and coils techniques.
With the much higher DC-link capacitance of a conventional VSD, the DC-link voltage will remain more stable under varying load and mains supply conditions, with a greater ability to absorb and store more energy. Under normal operating conditions, the DC-link ripple voltage of a conventional VSD with DC chokes remains relatively constant, rising from approximately 1% at no-load to 5% at full-load. In a Slim DC-link VSD the ripple voltage can rise as high as 15% at full load. It can also be expected that the DC-link voltage will fluctuate more widely under mains dips and dynamic motor load conditions (e.g. during acceleration and deceleration). This means that any VSD protection scheme based on the DC-link voltage or the DC-link ripple voltage is likely to be more sensitive in a Slim DC-Link VSD than a conventional VSD and more prone to frequent trips and that it cannot even detect some potentially damaging operating conditions.
Overall, the benefits of Slim DC link can be summed up as offering a slightly more compact design with same or lower harmonics content below 2kHz. However the drawbacks of Slim DC Link are many and some even severe: high voltage disturbances above 2kHz creating potential for high frequency resonance issues and casting doubt on the adherence to future harmonics standards, and high torque ripples on the motor shaft creating noise and vibrations and making it less suitable for dynamic applications.
Thus on balance the traditional DC-coil topology is still “best in class” when it comes to reliability and true harmonics performance in the lower / cost effective end of the spectrum of harmonics mitigation solutions today.
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Article by Ioannis Moles